Embodiments of the present invention generally relate to millimeter wave (MMW) imaging, and more specifically, illuminating a scene with diffuse radiation to improve the contrast produced by a passive MMW imaging sensor.
Electromagnetic energy in the millimeter wave region possess wavelengths which advantageously can be used to form images using relatively small imaging sensors, while being able to penetrate classes of materials which can commonly be used to conceal or camouflage weapons, containers, various other forms of contraband. In other applications, MMW imaging sensors may be used to detect people and/or animals by penetrating through walls. MMW imaging can offer real-time remote detection of hazards of metallic and/or non-metallic materials which can be concealed in a variety of ways, including being cloaked by one or more layers of clothing.
Either passive or active MMW imaging sensors may be used for concealed contraband detection; however, passive sensors can have the advantage of simplicity, reduced size, and reduced power consumption. Additionally, because passive MMW imaging sensors do not directly provide electromagnetic radiation for scene illumination, the passive sensors are not associated with detrimental physical side effects, or any perceived notions thereof, associated with the irradiation of subjects under surveillance.
Both passive and active MMW imaging sensors typically rely on contrast to generate useful images. The environmental conditions under which a passive MMW image created can be is one of the principal contributors in providing sufficient contrast. When imaging subjects outdoors, the clear sky can provide an excellent background for MMW imaging because it appears “cold” in the MMW wavelengths, that is, the sky provides little energy in the MMW region and therefore registers with a low temperature. The cold clear sky background can be exploited for detecting concealed objects.
An example of concealed object detection occurring outdoors on a clear day is as follows. A metal object such as a gun may be visibly concealed on a person under clothing, however, the surface of the gun which is not in contact with the person will “reflect” MMW energy corresponding to the cold sky, because clothing can be transparent to MMW waves. Moreover, the person will tend to have higher than ambient MMW temperature (as will be for fully described below), which can also be readily apparent to a MMW sensor given the transparency of the person's clothes. The difference in temperature between the person and the concealed gun provides sufficient contrast so the gun can be detected underneath the person's clothing.
However, when the above passive MMW imaging scenario occurs indoors, or in some other reduced contrast scenario such as outdoors on a rainy day there may not be sufficient contrast to reliably detect the concealed gun. While energy associated with MMW wavelengths can pass through walls depending upon the material from which they are made and wall's thickness, MMW contrast can be greatly attenuated indoors. Accordingly, the cold sky may not be available to provide sufficient contrast to detect the concealed gun or any other object behind the person's clothes. Like other forms of electromagnetic waves, MMW radiation can be polarized. MMW radiation sources and sensors can be configured to utilize polarization to distinguish between various types of materials. For example, objects made from metals will reflect polarized MMW radiation differently than objects made from plastics. These variations in reflected energy can be exploited with subsequent image processing techniques to provide additional information to aid in the classification of concealed objects.
In the past, various approaches have been tried to improve the ability of passive MMW sensors to make useful images in low contrast scenarios, but have been met with limited success. These approaches include providing MMW illumination sources such as oscillators, noise sources, and point sources, sometimes coupled with diffusers, all operating in the MMW region. These techniques can be problematic because they tend to saturate the MMW sensor, as it may have limited dynamic range. Moreover, images generated utilizing these MMW sources can suffer from glint and scintillation effects which can reduce the ability to exploit the MMW image to detect concealed objects. Other techniques have included providing a controlled environment, such as thermally controlled panels (e.g., a wall chilled using circulated water) to act as a cool radiometric background for improving contrast. Moreover, these approaches can increase complexity and cost as they may involve careful geometric control, precise signal timing, and/or critical alignment of scene being imaged.
What is therefore needed is a common, unobtrusive source that can provide diffuse MMW illumination in a simple and cost effective manner.